1. Field of the Invention
The present invention relates to a method and apparatus of optically detecting a beam of light such as laser beam and the like. More particularly, the present invention concerns a photodetection apparatus suitable for use in such a rotating laser system as known in the fields of survey, construction and civil engineering and a method of photodetection therein.
2. Description of the Related Art
Photodetection techniques for measuring the intensity of light, the position of projected light and others have been broadly utilized in various fields.
FIG. 5 shows an example of rotating laser systems for performing the leveling in the survey, construction and civil engineering. Such a rotating laser system comprises a laser beam projecting device 2 and a photodetection device 1 for receiving the laser beam from the laser beam projecting device 2.
The laser beam projecting device 2 emits a laser beam L turned about a vertical axis O to define a reference plane P. The photodetection device 1 is positioned on a vertical stationary plane such as wall or pile and comprises at least first and second light receiving elements 11 and 12.
If the laser beam L from the laser beam projecting device 2 is projected only onto the upper light receiving element 11, this means that a reference mark 5 in the photodetection device 1 is positioned below the reference plane P defined by the laser beam L. This position of the laser beam causes "upward direction" indicators 3b and 4b to light on. Each of these indicators 3b and 4b has an upward arrow indicating the necessary upward movement of the photodetection device 1. On the contrary, if the laser beam is projected only onto the lower light receiving element 12, this means that the reference mark 5 in the photodetection device 12 is positioned above the reference plane L. As a result, it causes "downward direction" indicators 3a and 4a each having an downward arrow to light on. The necessary downward movement of the photodetection device 1 is thus indicated. In addition, if the laser beam L is equally projected on both the upper and lower light receiving elements 11 and 12, it causes "center" indicators 3c and 4c to light on. This informs an operator that the reference mark 5 in the photodetection device 1 is brought into alignment with the reference plane P. The operator can obtain a leveling mark by drawing a line 1e on the wall or pile at the position of the reference mark 5.
Since the rotating laser device is frequently used out in the fields, the light receiving elements 11 and 12 will also receive the sun's rays as disturbance rays in addition to the laser beam L from the laser beam projecting device 2 which is used for measurement. When the sun's rays enter the light receiving elements 11 and 12, these elements will output a combination of its normal signals (pulsed signals) based on the laser beam L with DC noise signal components which are created from the reception of the sun's rays and superposed over the normal signals.
In order to overcome such a problem, the prior art photodetection device 1 comprises AC coupling circuits as composed of capacitors, which are connected with the respective light receiving elements 11 and 12. Each of the AC coupling circuits serves to eliminate the adverse affection of the sun's rays by removing the DC noise signal components and taking out only the pulsed DC components from the normal signals based on the laser beam L.
On the other hand, the rotating laser system may be utilized indoors. In this case, the photodetection device 1 will receive indoor illumination rays from any source of pulsed light such as fluorescent lamp. In general, the fluorescent lamp repeatedly turns on and off at a frequency of 100 Hz or 120 Hz or more. Thus, variations in the intensity of outputted rays cause pulsed AC components as shown by I.sub.n in FIG. 6(a), such components being mixed with the normal signals I.sub.s from the laser beam L.
In the prior art photodetection device 1, the AC coupling circuits can remove only DC noise signal components. As a result, the output signals from the AC coupling circuits contain a combination of the normal output signal I.sub.sc from the laser beam L with AC noise signals I.sub.nc from the illumination rays of the fluorescent lamp if the photodetection device 1 is used under the illumination of the fluorescent lamp, as shown in FIG. 6(b). The AC noise signals I.sub.nc will be also inputted into the processing circuit of the photodetection device 1. In the prior art, the processing circuit of the photodetection device 1 is not constructed so as to distinguish the AC noise signals I.sub.nc from the normal output signal I.sub.sc. This results in errors in operation and measurement.
The present invention is directed to provide a photodetection method and apparatus which can prevent any error in operation and measurement from being created from the influence of disturbance rays from a source of pulsed light such as fluorescent lamp or the like.